This invention relates to a method for diversity reception capable of maintaining a communication circuit with high reliability in a low-quality communication circuit and to an apparatus to be used for practicing the method.
Progress in the science of communication is expected to increase the reliability of communication in the transmission of information at great distances. In low-quality communication circuits such as in satellite communication and beyond-the-horizon communication, the signals received have a very low carrier-to-noise ratio (C/N) or desired-to-undesired signal ratio (D/U), and the signals, while propagating over great distances, are highly susceptible to fading, atmospheric interference and ratio interference. Consequently, the threshold level of received power is lowered even to a point where interruption of communication occurs frequently. Diversity communication has been developed to the level of practical utility with a view to improving the quality of received signals in the field of distance reception. Diversity communication aims to enable the reception of signals to be obtained in the highest possible condition by utilizing all possible devices involving use of a plurality of propagation paths or channels, adoption of specific techniques and choice of physical layouts, receiving one single signal through the medium of any of the aforementioned devices using two or more reception systems, selecting or combining the signal energies obtained in the plurality of reception systems thereby avoiding possible effects such as of fading to the utmost extent.
Broadly, diversity communication is divided into frequency diversity communication involving use of transmission signals of different frequencies, space diversity communication having a main and an auxiliary antenna installed at a proper distance from each other on the receiving unit side, polarization diversity communication involving variation in the plane of polarization, angle diversity communication involving variation in the direction, etc. In the corresponding various forms of diversity reception, the stability of the reception of signals is improved by providing their respective receiving units with a phase lock loop (hereinafter referred to as PLL) which is generally adopted for the purpose of minimizing possible effects of variations in the frequency of transmitted signals or variations in the locally oscillated frequency of the receiving unit. Even in diversity reception system utilizing PLL, however, the interruption of the reception as by atmospheric interference, radio interference or internal noise of the receiver cannot be completely eliminated. The reception of signals in the plurality of systems is stabilized by phase-locking the received signals at the central frequency thereof with the local oscillation signals which are voltage controlled in accordance with the phase difference between those of the received signals and the reference frequency signals. Despite the stability of signal reception described above, the reception of signal by any of the systems is interrupted when the phase-lock in the system is lost because of atmospheric interference or radio interference or because of internal noise of the receiver, for example. Upon this interruption, the circuit in such a system automatically actuates its own search functions to sweep the region of frequency in the neighborhood of the frequency of the received signal and resume phase-locking of the received signal in the system. If, however, the phase-lock in the PLL circuits of the remaining systems is lost while the search is in process prior to the resumption of the phase-lock, the reception of signals by the systems becomes totally ineffective.
To overcome these difficulties, there have been proposed techniques for facilitating the composition of received signals by use of a transmission signal possessing a coherent frequency (U.S. Pat. No. 2,951,152 patented Aug. 30, 1960, W. Sichak et al. "Radio Diversity Receiving System"; U.S. Pat. No. 2,955,199 patented Oct. 4, 1960, B. M. Mindes "Radio Diversity Receiving System"; and U.S. Pat. No. 3,195,049 patented July 13, 1965, F. J. Altman et al. "Radio Diversity Receiving System with Automatic Phase Control") and techniques for minimizing the probability of failure of phase-lock in the PLL circuit (U.S. Pat. No. 3,348,152 patented Oct. 17, 1967, C. R. Laughlin, Jr. et al. "Diversity Receiving System with Diversity Phase-Lock"). All these newly proposed techniques entail a requirement that, upon failure of the phase-lock in any of the PLL circuits of the plurality of systems, the remaining circuits should automatically actuate their own search functions and resume the phase-lock. However, none of these new techniques is found to possess a special function to shorten the time spent in the resumption of phase-lock.
An object of this invention is to provide a method for diversity reception which enables the communication circuit to be maintained in the condition of high reliability by permitting instantaneous resumption of the phase-lock upon failure of the phase-lock caused in some of the PLL circuits of the plurality of systems owing to atmospheric interference or radio interference or internal noise of the receiver and thereby minimizing the duration of the failure of the phase-lock, and an apparatus to be used for actually working the method described above.